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In real life
Otherwise known by it's chemical name of potassium nitrate, most commonly used in the production of medicines, fertilisers, and basic gunpowder (i.e. the most fun), which we will be dealing with here. It has been used along with the other component ingredients in this manner since the late 1700s.
A simple, good ole' fashioned recipe for gunpowder (proportions are by weight):
75% potassium nitrate
15% softwood charcoal
Which, when combusted, gives you the following reaction:
2 KNO3 plus S plus 3 C → K2S plus N2 plus 3 CO2
Molecular formula - KNO3
Density - 2.109 g/cm3 (16 °C)
Melting point - 334 °C
Boiling point - 400 °C
Solubility - in water 133 g/L (0 °C), slightly soluble in ethanol and also in glycerol and ammonia
In video games
A necessary resource that you need to gather in order to facilitate the creation of gunpowder units/technology in the Civilization series of games. This arms race is one any good warmongering civilization wants to win, as it allows domination over a vast sway your enemy's previous (lesser) units.